This invention pertains to mufflers. In general, a muffler has an outer shell, generally steel, which encloses a medium which absorbs and/or otherwise attenuates the sound emitted by e.g. an internal combustion engine. An inlet pipe feeds exhaust gases from the engine into the muffler. An exit pipe carries the exhaust gases away from the muffler.
The medium inside the muffler can be as minimal as the air which is inherently contained inside the muffler shell, namely the exhaust gases and/or shock waves pass/expand from the inlet pipe into the bulk of the shell cavity, and then pass from there into the exit pipe.
In other embodiments, the medium includes a multiple-pass path of pipes and baffles inside the muffler shell, where the multiple-pass path carries the exhaust gases through an elongate journey through the muffler, and where the length of the path, in combination with the internal pipe configuration, and other acoustic design properties, collectively contribute to sound attenuation in the muffler.
More commonly, the muffler shell is packed with a fibrous packing material such as fiberglass which may be separately fabricated as an “insert”. The exhaust gases, and/or the shock waves in the exhaust gases, are allowed and/or directed to flow into and/or through the fibrous packing material whereby the fibrous material absorbs/attenuates a portion of the sound.
This invention pertains, specifically, not only to mufflers in general, but also to muffler inserts, and methods and apparatus for fabricating muffler inserts.
In general, the process of reducing the intensity of the sound emitted by an engine, in a fiberglass-packed muffler, relates to the ability to disburse the sound waves into the medium materials so the medium can absorb and disburse the energy of the sound waves. While fiberglass is typically used as the fibrous packing material medium, other high temperature materials can be used in place of the fiberglass.
The muffler shell, which is packed with fiberglass, is also known as a canister. The inlet pipe, leading into the muffler, carries the exhaust gases from the engine into the muffler. The exit pipe, leaving the muffler, receives the exhaust gases which pass through the muffler, and passes those exhaust gases to downstream portions of the exhaust system. The exit pipe leaving the muffler may be an extension of the inlet pipe which carries the exhaust gases into the muffler. In the alternative, the exhaust gases may traverse one or more additional pipes inside the muffler whereby there may or may not be additional exhaust-gas carrying pipes and/or baffles inside the muffler shell, depending on the specifications of the particular muffler; and the exit pipe may not be the same pipe as the inlet pipe.
Some or all of the space inside the muffler shell, which is not occupied by the inlet pipe, the exit pipe, or any other internal structure inside the canister, is desirably occupied by uniformly packed fiberglass, which fiberglass provides a substantial portion of the sound attenuation properties of the muffler.
While some mufflers have a plurality of internal metal baffles and/or pipes which direct the exhaust gases in a tortuous path, other mufflers, as is the case in the embodiments illustrated, attenuate the sound in the exhaust gases as the pipe carrying the exhaust gases makes a straight-line pass through the muffler. The primary means for attenuating the sound in a straight-through muffler, such as in the embodiments illustrated herein, is to surround the inlet pipe, and/or another pipe inside the muffler, with a pack of fiberglass or other fibrous material. The fiberglass pack is surrounded by the outer shell such that the fiberglass pack is held between the outer shell and an exhaust-gas-carrying tube.
In some instances, the fiberglass insert is packaged in a plastic bag such that the plastic layer generally protects a worker's hands from the harsh affects of the fiberglass on human skin. When a muffler containing such insert is incorporated into an engine exhaust system, and the engine is activated, the heat from the exhaust gases melts and burns off the plastic bag, and at about 600 degrees F. sustained temperature, the gases also burn off any phenolic resin/binder in the fiberglass pack, leaving only the fiberglass as the “pack” inside the muffler. Once the fiberglass is released from any such binder as the binder and plastic film are burned off, the fiberglass, in general, expands to fill the space into which the insert was inserted, namely the space being occupied inside the muffler.
Restated, as a fiberglass pack is fabricated, certain transverse stresses are imposed on the individual strands of fiberglass. Those transverse stresses are maintained in the insert by the combination of any cured resin and any surrounding plastic bag. Once the bag and binder are burned off inside the muffler, the resilient nature of the transverse stresses, on the strands, cause the strands to move in transverse restoration directions until otherwise restrained by other strands, or by the muffler shell or pipes. Thus, when any binder and any bag are burned off, the fiberglass pack, as a whole, expands to a less-stressed condition, and correspondingly better fills the available space inside the muffler shell.
The efficiency with which a muffler attenuates sound depends in part on the uniformity of the density of the fiberglass in the fiberglass pack at steady state operation of the muffler. Uniformity of fiberglass density also influences uniformity of temperature distribution inside the muffler as well as at the muffler shell, thus effecting thermal stress distribution in the muffler, which influences use life of the muffler.
The extent to which the expanded fiberglass density is uniform throughout the available space inside the muffler shell, depends in part on the ability of the insert to conform to the available space, and in part on the uniformity of the density of the fiberglass in the insert as the insert is assembled into the muffler shell.
The problem addressed by the invention is that of creating a reproducible fiberglass insert which resides between the exhaust-gas-carrying tube and the outer muffler shell, starting with continuous fiberglass rovings as the raw material from which the insert is made and which provides desirably uniform density distribution of the fiberglass during steady-state operation of the muffler, while providing suitable safety to workers who install such inserts in the process of assembling mufflers.
Known processes by which fiberglass-based products are made and/or filled into muffler shells result in uneven distribution of the fiberglass inside the muffler shell, or distribution which is not reliably repeatable, such that, when the binder and/or plastic burn off, the fiberglass density is not reliably evenly distributed in the occupied space, which results in hot spots in the muffler, or there is variation from muffler to muffler, or from one production run to a subsequent production run.
Thus it is desirable to provide a method of uniformly distributing the fiberglass-binder mixture in an insert.
More specifically, it is desirable to provide a method of uniformly distributing a fiberglass-binder mixture in a mold which receives the fiberglass-binder mixture and provides a shape-constant core for the insert.
It is also desirable to provide a process by which uniformity and density of the fiberglass-binder core is reliably reproducible over an extended period of time.
Because of the known detrimental effects of fiberglass on human skin, it is also desirable to provide a method of fabricating a generally shape-constant muffler insert while providing a surface on the insert which limits the exposure of a worker's skin to the fiberglass used in the insert.
It is further desirable to provide a method of assembling a muffler product which includes assembling, into a muffler shell, a generally shape-constant muffler insert wherein a fiberglass-binder mixture is generally uniformly distributed in a core of the insert, and wherein a shrink film generally surrounds the outer surface of the core.
It is further desirable to provide a method of fabricating a muffler insert article, and muffler into which the insert article has been assembled, wherein the quality of the insert product is reliably reproducible.
It is yet further desirable to provide apparatus adapted to fabricate a fiberglass-based muffler insert and wherein the insert fabricated using such apparatus defines a generally uniform distribution of fiberglass throughout the volume defined by such insert, and wherein the insert is reliably reproducible.
It is further desirable to provide a cyclone mixer to mix fiberglass and binder, in cooperating combination with one or more funnels adapted to convey the fiberglass-binder mixture to a mold.
It is yet further desirable to provide a tamper which tamps the fiberglass-binder mixture into the mold, optionally through the funnel.
It is still further desirable to rotate the mold, either while the fiberglass-binder mixture is being conveyed from the mixer to the mold, or after a first charge of the mixture has been conveyed to the funnel and before a second subsequent charge of the mixture has been conveyed to the funnel.